Optical glass fibers are frequently coated with two or more superposed radiation-curable coatings which together form a primary coating immediately after the glass fiber is produced by drawing in a furnace. The coating which directly contacts the optical glass fiber is called the “inner primary coating” and an overlaying coating is called the “outer primary coating.” In older references, the inner primary coating was often called simply the “primary coating” and the outer primary coating was called a “secondary coating,” but for reasons of clarity, that terminology has been abandoned by the industry in recent years. Inner primary coatings are softer than outer primary coatings.
Single-layered coatings (“single coatings”) can also be used to coat optical fibers. Single coatings generally have properties (e.g., hardness) which are intermediate to the properties of the softer inner primary and harder outer primary coatings. The relatively soft inner primary coating provides resistance to microbending which results in attenuation of the signal transmission capability of the coated optical fiber and is therefore undesirable. The harder outer primary coating provides resistance to handling forces such as those encountered when the coated fiber is ribboried and/or cabled.
Optical fiber coating compositions, whether they be inner primary coatings, outer primary coatings, or single coatings, generally comprise, before cure, a polyethylenically-unsaturated monomer or oligomer dissolved or dispersed in a liquid ethylenically-unsaturated medium and a photoinitiator. The coating composition is typically applied to the optical fiber in liquid form and then exposed to actinic radiation to effect cure.
For the purpose of multi-channel transmission, optical fiber assemblies containing a plurality of coated optical fibers have been used. Examples of optical fiber assemblies include ribbon assemblies and cables. A typical ribbon assembly is made by bonding together a plurality of parallel oriented, individually coated optical fibers with a matrix material. The matrix material has the function of holding the individual optical fibers in alignment and protecting the fibers during handling and installation. Often, the fibers are arranged in “tape-like” ribbon structures, having a generally flat, strand-like structure containing generally from about 2 to 24 fibers. Depending upon the application, a plurality of ribbon assemblies can be combined into a cable which has from several up to about one thousand individually coated optical fibers. An example of a ribbon assembly is described in published European patent application No. 194891. A plurality of ribbon assemblies may be combined together in a cable, as disclosed, for example, in U.S. Pat. No. 4,906,067.
The term “ribbon assembly” includes not only the tape-like ribbon assembly described above, but optical fiber bundles as well. Optical fiber bundles can be, for example, a substantially circular array having at least one central fiber surrounded by a plurality of other optical fibers. Alternatively, the bundle may have other cross-sectional shapes such as square, trapezoid, and the like.
Coated optical fibers whether glass, or as has come into use more recently, plastic, for use in optical fiber assemblies are usually colored to facilitate identification of the individual coated optical fibers. Typically, optical fibers are coated with an outer colored layer, called an ink coating, or alternatively a colorant is added to the outer primary coating to impart the desired color.
Heretofore, the inner primary coating has not been colored. Coloring agents have not been included in the inner primary coating due to difficulties in obtaining proper cure of the inner primary coating in the presence of the pigments that have been used: commonly in the art to impart color to ink compositions.
Solvent-based, solid-based and pigment-based ink coatings have been used to impart color to coated optical fiber. However, none of these types of ink coatings is entirely satisfactory.
Solvent-based ink coatings are not satisfactory because solvent emissions cause environmental concerns. A further disadvantage of solvent-based systems is that finishing time is limited by the rate at which the solvent can be removed. Solvent removal rate is generally far too slow for high speed manufacture of optical fibers; Solid-based coloring systems are also too slow.
Ultraviolet curable pigment-based inks are described, for example, in U.S. Pat. No. 4,629,285. Pigment-based ink coatings are formed from a pigment dispersed within a UV curable carrier system. The UV curable carrier system contains a UV-curable oligomer or monomer that is liquid before curing to facilitate application of the ink composition to the optical fiber, and then turns to a solid after being exposed to UV radiation. In this manner, the UV-curable ink composition can be applied to a coated optical fiber in the same manner as the inner primary and outer primary coatings are applied.
Pigment-based ink coatings are not entirely satisfactory because the pigment particles tend to act as an internal reflector, causing scattering of the curing radiation and also tending to absorb actinic radiation. As a result, radiation is prevented from penetrating the entire thickness of the coating, thereby extending curing time, or limiting thorough cure. While photoinitiators which absorb light at a different wavelength than the polymer have been used in pigment-based ink coatings to improve thorough cure and cure time, the scattering problem still remains.
Further, pigment-based ink compositions are not useful to impart color to either the inner primary coating or to matrix materials. Inclusion of pigments in the inner primary coating can abrade the glass or plastic waveguide of the coated optical fiber. Matrix materials are thicker than the inner primary coating or outer primary coating, and inclusion of pigments in the matrix materials can impair thorough cure of the matrix materials.
Despite the efforts of the prior art to provide materials and methods to impart color to coated optical fibers, there remains a need for a method of achieving the color necessary to facilitate identification of the individual coated optical fibers or of other desired components while satisfying the many diverse requirements desired, viz., improved curing and enhanced cure speeds and versatility in application while still achieving the desired physical characteristics of the various coatings employed. There also remains a need for an inner primary coating composition which is colored.